I have a 9 month old and a three year old and we are planning on getting a roof top tent. So far, the kids sleep next to us in the ground tent just fine with no extra accessories. I use a travel crib mattress without the travel crib for the baby and a regular sleeping pad for the toddler. Our plan is to put one parent and one child in the rtt and one parent and one child in the back of the van. We are trying to streamline the whole thing as much as possible because if I'm being truthful, loading everyone up to go to the grocery store is a bit of a chore!
Also, Colorado camper vans makes a pretty cool roof top tent with an optional hanging cot for little ones. I'd go for that, but their tents are a little too narrow for my liking. If your baby co-sleeps in a ground tent, I can't see any issues with a rtt. They are pretty adaptable little creatures. Good tip on the crazy creek for nursing. I've been using a helinox in the ground tent.
Also, Colorado camper vans makes a pretty cool roof top tent with an optional hanging cot for little ones. I'd go for that, but their tents are a little too narrow for my liking. If your baby co-sleeps in a ground tent, I can't see any issues with a rtt. They are pretty adaptable little creatures. Good tip on the crazy creek for nursing. I've been using a helinox in the ground tent.